Essentially all continuous digesters for producing pulp from cellulosic chips material contain screens for withdrawing liquor from the cooking portions of the digester, heating the withdrawn liquor to control the temperature thereof, and returning the liquor to the cooking portion of the digester. Numerous different types of proposals have been made for the exact manner of effecting the liquid withdrawal, such as shown in U.S. Pat. Nos. 2,695,232; 3,711,367; and 4,061,193. Many conventional commercial digesters have two upper cooking circulation screens and two lower cooking circulation screens, with associated headers, with one outlet extending from each header.
While conventional techniques for effecting cooking liquor recirculation are normally satisfactory, in larger digesters pulp uniformity and quality sometimes suffer as a result of problems in digesting liquor circulation. This is particularly true for digesters having a diameter of about 14' or more.
According to the present invention, the sources of the problems with pulp quality and uniformity as a result of cooking liquor recirculation have been identified, and pulp uniformity and quality have been improved in a simple, economical and common effective manner. It has been recognized that as the distance from the central distribution pipe for the cooking liquor recirculation to the digester wall increases, it is easier for the liquor to find its own path as it moves through the chips near the wall area. It has been further determined that as the distance around the liquor collection headers under the screens is increased it becomes increasingly more difficult to compensate for pressure drops down the header as more liquor is picked up from the bottom of each screen, which problem is accentuated as the header scales up. It has been further determined that in the period between acid cleanings, the headers gradually plug up with accumulated scale and debris, and this accumulation is nonuniform and is also worse at the point of lowest velocity. Finally, it has been recognized that because of scaling, screens are normally installed that have much more screen area than is ncessary to support the design flow; when the screens are clean, the liquor can choose its own path because of larger area available, and as the screens scale the scaling and deposits prevent an even flow distribution, thus there is always some problem with distribution whether the screens are clean or scaled up. This flow distribution problem is further aggravated since the screen area varies directly as the diameter, whereas the tonnage (and required circulation flow) varies as the square of the digester diameter, and for larger digesters the only way to hold the loading per unit screen area down would be to make the screens very tall, which involves considerable expense for both the screens and the digester shell.
According to the present invention, the flow distribution problems that result in decreased product unformity and quality have been solved in a simple and efficient manner. According to the present invention, instead of providing one liquor outlet with each header a pair of circumferentially spaced (e.g., 180.degree.) outlets are provided with each header, no restrictions being provided in the headers preventing flow from one portion of the header to another; and a method of withdrawing liquor from the outlets is provided that ensures that a header of each header pair is rested during each alternate withdrawal step, and that the side from which liquor is withdrawn is alternated each successive time that a header is used. This varies the position from which the liquor is extracted and ensures that no paths are set up inside the digester where liquor is always pulled only one way, this being particularly important as the screen starts to scale over. By switching from one side to the other, the low velocity point in the headers is moved from cycle-to-cycle and all points of the header are exposed to a high velocity at least once per complete cycle. This keeps debris and scaling from occurring in the section of the header which was previously only exposed to low velocity liquor, keeping the header much cleaner and preventing any channelling effects due to header blockage caused by deposits, debris and scale from originating. In comparative tests of the switching apparatus and method according to the invention and conventional arrangements utilizing only one liquor withdrawal outlet per header, it was found that increased uniformity and improved product quality unmistakably resulted from the practice of the invention.
In particular, according to the present invention a method of withdrawing liquid from a pair of vertically spaced annular screens in a vertical vessel is provided. Each screen has an annular header associated therewith and two circumferentially spaced liquid outlets are associated with each header, so that first and second headers each having first and second outlets are provided. The method comprises the steps of substantially sequentially withdrawing liquid from the first header first outlet; terminating withdrawal of liquid from the first header first outlet and withdrawing liquor from the second header first outlet; terminating withdrawal of liquid from the second header first outlet and withdrawing liquid from the first header second outlet; terminating withdrawal of liquor from the first header second outlet and withdrawing liquor from the second header second outlet; and repeating the above steps with appropriate frequency to minimize header blockage caused by deposit of debris and scale. Preferably, the first outlets of the first and second headers are somewhat circumferentially spaced from each other, and the second outlets of the first and second headers are somewhat circumferentially spaced from each other. Also, preferably a second pair of annular screens with associated headers and liquor outlets are provided vertically spaced from the first pair, with a withdrawal sequence from the second pair liquor outlets substantially coincident with the withdrawal from the first pair corresponding liquor outlets.
Further according to the present invention, a continuous digester for cellulosic pulp material is provided including a vertically extending elongated vessel; a pair of circumferentially spaced annular screens disposed in the vessel, each having an annular header associated therewith; a first liquid outlet associated with each header; means for withdrawing liquid from the vessel operatively connected to each outlet; means for heating the withdrawn liquid and returning it to the interior of the vessel; and means for selectively providing withdrawal from a given outlet while preventing withdrawal from the other outlets. The improvement comprises a second liquid outlet provided with each header and circumferentially located approximately 180.degree. from the first outlet provided with each header, the second outlets operatively connected to the liquid withdrawing and selective withdrawal providing means so that liquid is withdrawn from only one said outlet at a time while withdrawal from the other said outlets is prevented. Such apparatus is particularly effective in the digester having a diameter of about 14' or greater.
It is the primary object of the present invention to provide a method and apparatus for the withdrawal and recirculation of cooking liquor in continuous digesters to minimize header blockage caused by deposited debris and scale, and to increase pulp uniformity and quality. This and other objects of the invention will become apparent from an inspection of the detailed description of the invention, and from the appended claims.